For the purpose of rolling metal products in band form, use is made of rolling mill trains, each of which consists of a stand comprising two spaced support columns which are connected by crosspieces and between which is mounted an assembly of superposed rolls whose axes are parallel and which are disposed in the same plane. Each roll is mounted for rotation about a shaft carried at its two ends by support means housed respectively in the two columns of the stand.
Different types of rolling mills can be made. Rolling mills known as "four-high" comprise four rolls, namely two work rolls, between which the rolled product passes and which are each supported, on the side opposite the product, on a backup roll. Other arrangements can also be envisaged, for example the passing of the band between a work roll and a coacting roll supported on the stand, its work roll being applied, on the opposite side, against a backup roll to which the pressure load is applied. Rolls of the "six-up" type, on the other hand, include intermediate rolls interposed between the backup rolls and the work rolls.
The rolls bear against one another along substantially parallel support lines directed along a generatrix whose normally rectilinear profile depends on the loads applied and the resistance of the roll. The pressure load is generally applied by screws or jacks interposed between the stand and the ends of the top backup roll shaft, while the bottom backup roll or coacting roll is supported directly on the stand. Apart from the last-mentioned roll, the other rolls must therefore be able to move slightly relative to the stand, and they are therefore carried by support means mounted for vertical sliding movement inside the corresponding columns of the stand.
In view of the fact that the pressure load is applied to the two ends of the top backup roll and that the rolled product, of variable width, does not cover the entire length of the work rolls, each roll can sag through the action of the loads applied, and this results in a variation of the thickness of the space for the passage of the band between the work rolls, so that the edges of the band may be thinner than the central portion.
For a long time, attempts have been made to correct these defects in thickness on the profile across the rolled product, and different means are employed for this purpose.
In particular, it has been proposed to utilize backup rolls having a slightly cambered external profile adapted to compensate for the sagging of the roll through the action of the pressure load, so that the generatrix of contact with the corresponding work roll will be substantially rectilinear. However, this profile can be determined only in dependence on an average rolling load.
Correction facilities are improved by using variable camber rolls comprising a central shaft on which is mounted a deformable cylindrical casing whose profile can be determined by applying thrust loads between the shaft and the casing in order to compensate for the sagging of the shaft in dependence on the rolling load.
In one known arrangement, the central shaft, which is mounted for rotation about its axis, is provided on its periphery with a space formed on the outside by the deformable casing, the latter being sealingly connected to the shaft in such a manner as to form a closed annular space into which a fluid under adjustable pressure is introduced.
By inflating the casing, it is thus possible to vary its profile.
In another known arrangement, which is used in particular in the paper industry, the peripheral wall of the backup roll consists of a tubular casing mounted for rotation about two bearings on the central shaft, the latter being secured against rotation and forming a support beam on which are supported a plurality of casing carrier shoes distributed along a generatrix of support and extending radially between the support beam and the inner face of the casing, with the interposition of a lubricant film permitting the rotation of the casing. Each shoe bears on the support beam with the aid of at least one jack allowing individual adjustment of the radial position of the shoe in dependence on the profile to be given to the line of support in order to correct defects in the thickness and flatness of the band, measured downstream of the rolling mill by known devices. The corrections can thus be distributed over the width of the band in such a manner as to achieve rapid correction of localized thickness or flatness defects.
Since the aim of the rolling operation is to obtain products whose thickness is as constant as possible, it is logical to use symmetrical installations comprising on each side of the product a variable camber roll whose profile is adjusted to compensate for the sagging of the shaft, in such a manner that the line of support on the corresponding work roll will be as rectilinear as possible. However, complete compensation for this sag cannot always be achieved, and in addition the work rolls, which are generally of small diameter, may tend to ovalize through the action of the loads applied. For that reason it has also already been proposed to supplement the correction of the profile of the backup rolls, achieved through a cambering action of the work rolls, by applying bending loads to the ends of the shaft of each roll.
Each work roll is in fact carried at its ends by bearings housed in chocks mounted for sliding in the corresponding columns of the stand, the relative positioning of which chocks can be adjusted by means of hydraulic jacks applying thrust loads in opposite directions to lugs fastened to each chock. A so-called "positive" camber can thus be achieved by reducing the space between the chocks of the two work rolls in order to compensate for increased thickness of the edges of the product, or a so-called "negative" camber can be obtained by moving apart the chocks of the two work rolls to compensate for increased thickness of the central portion of the product.
The invention is therefore particularly, though not exclusively, applicable to a rolling mill of the four-high type comprising, inside a support stand, four rolls having parallel axes and superposed along a roll adjustment plane, each mounted for rotation about a shaft carried at its two ends by support means housed in the columns of the stand, i.e., two work rolls forming between them an elongate space for the passage of the product and each bearing on the opposite side against a backup roll, at least one of which backup rolls is of the variable camber type comprising a cylindrical casing mounted for rotation on a central shaft, and means for adjusting the profile of the casing through the application of thrust loads between the central shaft and the inner wall of the casing, the pressure load being applied to the ends of the backup roll shaft and the work rolls being associated with cambering means applying bending loads to the ends of their shafts.
For the purpose of checking the flatness and uniformity of thickness of the rolled product under the best possible conditions, it has previously been considered normal to make use of symmetrical installations comprising in particular two variable camber backup rolls. However, arrangements of this kind are expensive and make use of delicate components liable to deteriorate, so that the cost of production and operation of the rolling mill is substantially increased.